c# 倒序读取txt文件-蒲公英云

c# 倒序读取txt文件

倒序读取txt文件，基本思路：
1.使用 FileStream 文件流，可以设置流的位置 position；
2.每次读取一行：行的标记就是两个 \n 之间的内容,占位符在流中的值为；
byte n = 0xD;//回车符
byte r = 0xA;//换行符
3.读取时每一行从后向前读，所以输出的字符串为倒的，故读完一行后要将内容反转；

4.倒序读行时同理，行的记录也是倒的，需要将行记录反转；

调用方法：
1.获取文件夹下的所有文件信息
var files = GetFile(“c:\logs”);
2.获取指定文件最后200行记录
var result = LastLog(“c:\\logs\log.txt”,0);
result.data ;//文件内容
result.position; //本次读取流的起始位置

主要方法：

/// <summary>
/// 获取目录下所有文件信息
/// </summary>
/// <param name="path"></param>
/// <returns></returns>
private List<LogFileModel> GetFile(string path)
{
    List<LogFileModel> list = new List<LogFileModel>();
    var files = Directory.GetFiles(path);
    if (files.IsNullOrDbNull())
        return null;
    LogFileModel item;
    FileInfo f;
    Func<long, string> getFileSize = (x) =>
    {
        if (x <= 0)
            return "0KB";
        var d = Math.Round(x*1.0/1024/1024, 2);
        return string.Format("{0}KB", d);
    };
    var fileInfos = new List<FileInfo>();
    files.All(x =>
    {
        fileInfos.Add(new FileInfo(x));
        return true;
    });
    int i = 1;
    //按照创建时间倒序
    fileInfos
        .OrderByDescending(x=>x.CreationTime)
        .ToList()
        .ForEach(x =>
    {
        item = new LogFileModel
        {
            name = x.Name,
            size=x.Length,
            desc = getFileSize(x.Length)
        };
        list.Add(item);
        if (i++ >= 20)
            return ;
    });
    return list;
}
/// <summary>
/// 倒序读取文本文件
/// </summary>
/// <param name="file">文件路径</param>
/// <param name="position">读取流的起始位置</param>
/// <returns></returns>
public LastLogResult LastLog(string file, long position)
{
    if (!File.Exists(file))
    {
        result.msg = "文件路径无效！";
        return result;
    }
    StringBuilder txt = new StringBuilder();
    long ps = position;
    using (var fs = new FileStream(file,FileMode.Open,FileAccess.Read))
    {
        if (ps <= 0)
            ps = fs.Length;
        //读取200行
        for (int i = 0; i < 200; i++)
        {
            ps = InverseReadRow(fs, ps, ref txt);
            if (ps <= 0)
                break;
        }
    }
    result.data = txt.ToString();
    result.position = ps;
    result.success = true;
    return result;
}
/// <summary>
/// 从后向前按行读取文本文件，最多读取 10kb
/// </summary>
/// <param name="fs"></param>
/// <param name="position"></param>
/// <param name="s"></param>
/// <param name="maxRead">默认每次最多读取10kb数据</param>
/// <returns>返回读取位置</returns>
long InverseReadRow(FileStream fs
    , long position
    , ref StringBuilder s
    , int maxRead = 10240)
{
    byte n = 0xD;//回车符
    byte r = 0xA;//换行符
    if (fs.Length == 0)
        return 0;
    var newPos = position - 1;
    int curVal = 0;
    var readLength = 0;
    List<byte> arr = new List<byte>();
    var str = "";
    //bool brk = false;
    do
    {
        readLength++;
        if (newPos <= 0)
            newPos = 0;
        fs.Position = newPos;
        curVal = fs.ReadByte();
        if (curVal == -1)
            break;
        arr.Insert(0, (byte)curVal);
        if (newPos <= 0)
            break;
        //
        if (readLength == maxRead)
            break;
        if (curVal != n)
            newPos--;
    } while (curVal != n);
    str = Encoding.UTF8.GetString(arr.ToArray());
    s.Insert(0, str);
    return newPos;
}

使用的对象

public class LogFileModel
{
    public string name { set; get; }
    public long size { set; get; }
    public string desc { set; get; }
}
public class ResultModel<T>
{
    public ResultModel()
    {
        data = default(T);
    }
    public string msg { set; get; }
    public bool success
    {
        set;
        get;
    }
    public T data { set; get; }
}
public class LastLogResult : ResultModel<string>
{
    public long position { set; get; }
}